1. Field of the Invention
The present invention relates to a preloader for use in a webbing retractor and adapted to allow a webbing to be applied to a vehicle occupant with appropriate tightness at the time of a sudden deceleration of a vehicle.
2. Description of the Related Art
A seat belt apparatus is adapted to restrain a vehicle occupant with appropriate tightness by means of a webbing at the time of a sudden deceleration of a vehicle. If there is a large clearance between the webbing and the occupant, however, the seat belt apparatus cannot display a restraining performance sufficiently. For this reason, preloaders for allowing the webbing to be applied closely around the occupant by forcibly pulling the webbing in the direction of retraction at the time of a sudden deceleration of the vehicle, as well as webbing retractors equipped with the preloader, have been proposed.
The preloader of this type is so arranged as to allow the webbing to be retracted and paid out freely without hampering the rotation of a takeup shaft, during the normal running of the vehicle. At the time of a sudden deceleration of the vehicle, however, the preloader must instantaneously rotate the takeup shaft in a taking-up direction so as to retract the webbing immediately. For this reason, the preloader of this type is provided with a clutch mechanism, so that a clutch is engaged only at the time of a sudden deceleration of the vehicle, thereby allowing the takeup shaft to rotate in the taking-up direction.
One known clutch mechanism is arranged such that a wire is wound with play around a rotating drum disposed integrally with the takeup shaft, and as the wire is forcibly pulled, the rotating drum is tightened by the winding force, and the rotating drum is rotated by a frictional force occurring during the tight winding so as to transmit the retracting torque of the preloader to the takeup shaft (Japanese Patent Application Laid-Open No. 59-95068 etc.).
With the clutch mechanism of this type, however, there is a drawback in that the wire can loosen and slip over the peripheral surface of the rotating drum, since the connection between the wire and the rotating drum utilizes the frictional force.